Only since 1800, in the last 0.1% of the history of Homo sapiens, has the human population shot into the billions. Now at nearly 6.7 billion, with 9 billion looming 40 years away, few environmentalists seem to care.

Yet the population-environment link is clear. Our environmental impact, as gauged by total resource consumption for a country or the world, is the product of population size and the average person's consumption.

Today's crumbling environment, racked by climate change, mass extinction, deforestation, collapsing fisheries and more is evidence our total consumption has gone too far. We are destroying our life-support system. In ecological terms we are in "overshoot" of Earth's "carrying capacity" for humans, our demand exceeding the planet's absorptive and regenerative capacities.

To avert catastrophe, we need to reduce both factors in the equation: our numbers and per person consumption.

Or so it would seem. Ignoring that logic, most environmentalists today avoid half the equation. An emailer's assertion was typical: "John, if everyone on Earth just consumed less, as they do in Mexico, say, we wouldn't have exceeded carrying capacity."

It's a simple notion: reduce per person consumption and end our environmental problems. And it lets us sidestep the issue of population size and growth, a subject of much concern in the 1960s and 1970s but taboo today.

Why taboo? Much credit goes to pressure from social justice activists. They've insisted in recent decades that any focus on numbers inevitably violates the right of women to manage their own fertility.

China's one-child policy notwithstanding, humane, successful population programmes in countries as varied as Thailand, Iran, and Mexico contradict that assertion.

Nevertheless, the criticism has cowed environmentalists and NGOs which once championed the population cause, influencing policy, pushing the subject off the agenda, or shifting the emphasis solely to "reproductive health" without the numbers.

Looking then for a way around the problem of growing human numbers, most environmentalists now suggest a reduction in individual consumption is all we need to solve our ecological problems.

Are they right? The work of the Global Footprint Network (GFN), home of the "ecological footprint," points to the answer. Measuring consumption as the use of biologically productive land and sea, their data shows a global maximum sustainable footprint, at today's population, of just under 1.8 global hectares (gha) per person. Currently, by drawing down nonrenewable resources, we're a bit over 2.2gha, overshooting Earth's limits by about 25%.

What if everyone took the emailer's advice and converged on Mexico's level of per capita consumption? Resource use would plummet in developed countries while rising in many of the poorest. (Surely we could not deprive the latter of the chance to raise their standards of living?) But it wouldn't get us to 1.8gha. At 2.6gha, Mexico's footprint is 32% too high. A drop to the level of Botswana or Uzbekistan would put us in the right range.

But that's not low enough. We'd next have to compensate for UN projections of 40% more humans by the middle of the century. That would mean shrinking the global footprint to under 1.3gha, roughly the level of Guatemala or Nigeria.

There's more. The GFN authors point out their data is conservative, underestimating problems such as aquifer depletion and our impacts on other species. In response, the Redefining Progress group publishes an alternative footprint measure which has humanity not at 25%, but at 39% overshoot. But that too, the authors concede, is an underestimate.

While in overshoot, moreover, we erode carrying capacity. Once we'd got to some level of consumption on a par with countries living today in abject poverty, we'd find there were fewer natural resources on which to draw than there had been when we started.

Ultimately, there are limits to how much we can reduce per-person use of land, water, and other resources. A purposeful drop on the part of industrialised countries to consumption levels comparable to those of the poorest areas in the world is not only wholly unrealistic but, at today's population size, would not end our environmental woes. Our sheer numbers prevent it.

We have no alternative but to return our attention to population, the other factor in the equation. Already in overshoot, we must aim for population stabilisation followed by a decline in human numbers worldwide.

Humane, empowering measures have documented records of success at reducing fertility rates. Most importantly, we have to provide easy access to family planning (pdf) options while educating parents through the media in the benefits of smaller families and family planning. We should educate and empower girls and women to give them options and help free them to make decisions concerning family size. And we should end government incentives for larger families. We must do these things internationally and vigorously, with a keen eye toward numbers, monitoring results and making adjustments accordingly.

The stakes are too high to waste time evading the issue. Doing so is intellectually dishonest and a setup for global tragedy. It's time environmentalists ended the silence on population.